1. Field of the Invention
This invention relates to electronic circuits, and more particularly, to amplifier circuits.
2. Description of the Related Art
Amplifiers having common mode feedback (CMF) are used in a wide variety of operational amplifier designs. Operational amplifiers with common mode feedback typically include a differential input stage and an output stage. The input stage provides the gain for the amplifier, along with other desirable operational amplifier characteristics such as low offset voltage and high common mode rejection. The output stage provides additional power gain to drive the output signal, and also provides the function of converting the differential input signal into a single-ended output signals.
In some amplifiers the input stage converts the differential input signal to a single output connection to the input stage and CMF is not required. In other cases, such as very wide input common mode range, the output is differential, and CMF is used to control the DC bias voltage of the high impedance node where the input stage and output stage are connected together. Controlling the DC bias voltage controlled by the CMF is important for maintaining all of the active devices in the proper operating range, and also determines the idling current (sometimes called quiescent current) in the output stage.
FIG. 1 is a schematic diagram of one embodiment of an operational amplifier output having differential inputs and a single-ended output. Output stage 10 includes two terminals, In+ and In−, that may be coupled to differential output terminals of a gain stage. A single-ended output (Out) is provided in order to couple the amplifier circuit to a load circuit. Although the embodiment here utilizes complementary metal oxide semiconductor (CMOS) transistors, embodiments utilizing bi-polar transistors are also possible.
FIG. 2 is a schematic diagram of one embodiment of a gain stage for an amplifier circuit having differential inputs. Gain stage 15 includes a pair of output terminals (Out+ and Out−) that may be coupled to the input terminals of an output stage such as the one illustrated in FIG. 1. In order to control the voltage at the output terminals, common mode feedback is employed. This also determines the current through transistors M5 and M6. The common mode feedback loop includes the junction of R1, R2, and the gate terminal of M3. Using the common mode feedback loop forces the output currents from M5 and M6 to be equal to the output current of M1 and M2, respectively. The CMF also forces the DC voltage at Out+ and Out− to be equal to the Vgs of M9. It should be noted that the gate terminals of M1 and M2 serve as differential signal inputs for the gain stage of circuit illustrated in FIG. 2.
During the operation of an amplifier such as one combining the circuits of FIG. 1 and FIG. 2, it is important to carefully control the bias currents and voltages in the amplifier stages. An operational amplifier often times need to have a voltage swing that is very close to the positive power supply and the circuit ground. This may necessitate an additional requirement of controlling the bias current in the output stage when the input signal is zero or near zero while coupling the driving signal to the to the output. If the bias control is not properly designed, the transistors can enter the linear region (for a CMOS implementation) of operation, and thus the amplifier circuit may no longer provide the desired gain to the signal. Because the DC voltage at nodes Out+ and Out−of FIG. 2 are equal to the gate to source voltage of M9, the gate to source voltage of transistors M1 and M4 of FIG. 1 also have the same gate to source voltage as M9. This allows control of the bias current in M1 and M4 of FIG. 2 by designing them to be scaled replicas of M9.